A base station transmitter in a mobile communications system transmits a high-power-level downlink signal to the mobile terminal within its coverage area. Generally, the base station transmitter converts a digital downlink signal to be transmitted to separate baseband signals containing amplitude and phase information, which are then amplitude and phase modulated onto a low-power-level RF signal. The resultant low-level RF signal is then amplified to a high power level for transmission via an antenna. Since the low-level RF signal is both amplitude- and phase-modulated, the high-power-level RF signal transmitted by the base station needs to be linearly related to the low-level RF signal in order for a mobile terminal to recover the modulating amplitude information. Conventionally a chain of class A linear amplifiers have been used to amplify the low-level RF signal, with a class AB amplifier being used at the output stage to provide the input to the transmitting antenna. It is well known, however, that linear amplifiers are costly and highly power inefficient, requiring large power supplies and associated cooling equipment. In order to minimize base station costs and size, therefore, a high efficiency (i.e., low loss) amplifier is desirable.
In order to improve efficiency, base stations in TDMA mobile communication systems have employed a well-known Kahn envelope-elimination-and-restoration technique for power-amplifying the RF signal for transmission. In accordance with this technique, digital amplitude information and phase information in the input signal are separated. A baseband signal derived from the digital phase information is modulated onto an RF carrier and amplified by non-linear amplifiers, which are quite efficient. An RF power amplifier then amplifies the phase-modulated carrier and modulates the amplitude of that RF signal with a baseband signal derived from the digital amplitude information. Before being supplied to the RF power amplifier, however, that amplitude-modulating signal is amplified by a linear amplifier. Typically, a linear amplifier that is more efficient than a class AB amplifier is used to amplify this signal such as, for example, a class D or a class S switching amplifier.
For a TDMA signal having a bandwidth of approximately 30 kHz, the switching frequency of such a switching amplifier needs to be approximately 300 kHz, which is within the capabilities of conventional prior art switching amplifiers. A switching amplifier that operates at that speed has a dynamic range (i.e., the capability of the amplifier to produce an output having a number of distinct and distinguishable output levels corresponding to the separate and distinct possible input levels, expressible in terms of dB or of bits of resolution) that is high enough for such TDMA signals. In a CDMA mobile communication system, however, the CDMA signal has a bandwidth that is standardized in current systems at 1.25 MHz. For use in a CDMA transmitter, a switching amplifier would need to operate at a switching frequency of at least 5 MHz and have a dynamic range of at least 72 dB for 12 bits of resolution, which is beyond the capabilities of prior art switching amplifiers.
A need exists, therefore, in a CDMA transmitter for a linear amplifier that has a wide bandwidth and high dynamic-range.